Bug 18579: Make EmbedItemsAvailability handle control fields correctly

Bug 18579: Make EmbedItemsAvailability handle control fields correctly

This patch makes the EmbedItemsAvailability filter handle the biblio.biblionumber > control field mapping gracefully.
Right now, it was assuming a regular data field was used, and such is not the case for UNIMARC (001).

To test:
- Apply the tests patch
- Run:
  $ sudo koha-shell kohadev
 k$ cd kohaclone
 k$ prove t/db_dependent/Koha/Filter/EmbedItemsAvailability.t
=> FAIL: Tests fail to run
- Apply this patch
- Run:
 k$ prove t/db_dependent/Koha/Filter/EmbedItemsAvailability.t
=> SUCCESS: Tests pass!
- Sign off :-D

Sponsored-by: ByWater Solutions
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>